Transmit diversity is beneficial because it mitigates degradation of the forward link due to signal fading. In its simplest sense, transmit diversity can be thought of as the transmitting a signal to a receiver using multiple antennas. In general diversity can be implemented at either the transmitter, the receiver or at both the transmitter and receiver. Additionally, diversity methods can be combined with feedback from the subscriber to the transmitter. Diversity methods with feedback are referred to as closed-loop methods, while diversity methods without feedback are referred to as open-loop methods. Open-loop transmit diversity methods are desirable because these methods have the least impact on subscriber complexity.
The three open-loop transmit diversity methods that have been included in the third generation standards are orthogonal transmit diversity (OTD), space-time spreading (STS), and space-time transmit diversity (STTD). The first two methods, OTD and STS, have been included in the current release of the third generation partnership project interim standard 2000 (IS20003GPP2 standard), while STTD is part of the third generation partnership project for wideband CDMA (3GPP standard).
Current methods for receiving CDMA signals involve the use of a matched-filter receiver, sometimes referred to as a RAKE receiver. More recently, however, it has been proposed to utilize a Minimum Mean Square Error (MMSE) receiver for reception of CDMA signals. MMSE receivers operate to minimize the mean-square error of the symbol estimates. A detailed description of the operation of such receivers for non-transmit diversity applications is given in the paper “Adaptive Interference Suppression for Direct-Sequence CDMA Systems with Long Spreading Codes” (Colin D. Frank and Eugene Visotsky, In Proceedings of the Allerton Conference on Communications, Control and Computing, Monticello, Ill., Sep. 23-25, 1998, pp. 411-420) Although MMSE receivers have been described for receiving CDMA signals in a non-transmit diversity environment, there currently exists no MMSE receiver capable of receiving CDMA signals with transmit diversity. Therefore, a need exists for an MMSE receiver capable of receiving DS-CDMA signals transmitted utilizing transmit diversity.